1. Field of the Invention
The present invention relates to a constant velocity universal joint which is applied to the axle shaft of an automobile of four-wheel drive type. In particular, it relates to a double Cardan universal joint.
2. Description of the Prior Art
In general, two yoke members of a double Cardan joint are connected with each other by a centering assembly. The centering assembly is necessarily exposed to continuous friction during transmission of torque. Therefore, the centering assembly generally has an inner space for containing grease therein for lubrication purpose and a breather hole for making the inner space communicate with the atmosphere. The breather hole is also adapted to adjust the internal pressure of the inner space by releasing air from the inner space into the atmosphere when the internal pressure is raised by injection of grease, angular motion of the centering assembly or temperature rise. An especially important function of the breather hole is to prevent deformation of the lip of a sealing member caused by impact pressure upon injection of the grease and displacement of the sealing member.
However, it has been apprehended that muddy water enters the inner space through the breather hole to deteriorate the grease and debase lubrication of the centering assembly depending on the condition under which the joint is used.